Stir the harissa into mayonnaise in a small bowl; taste for spiciness. Add more harissa or more mayonnaise according to taste; set aside.

Prepare a grill for medium heat. Meanwhile, combine the jicama, avocado and onion in a bowl; stir in the lemon juice, olive oil, 1/4 teaspoon of the salt, and pepper to taste.

Season the chicken with remaining 1/4 teaspoon of the salt and pepper to taste; grill, turning as needed, until done, about 5 minutes per side. Cut chicken into small dice, or shred (cubes will be faster; shredding gives better texture). Warm tortillas on the grill until softened, taking care not to burn.

Divide chicken among tortillas; top with about 2 tablespoons of jicama mixture. Add 1/2 teaspoon harissa mayonnaise or to taste; top with cilantro. Fold tortilla around filling.